Measurement of acoustic intensity has been performed for evaluation of noise or analysis of sound propagation in a concert hall. Unlike acoustic pressure which is a scalar quantity, the acoustic intensity is a vector quantity and includes not only information such as volume, frequency, and waveform of sound from a sound source but also information about direction of sound. That is, not only the volume of sound but also the information as to which direction sound is traveling from can be measured based on the acoustic intensity.
The acoustic intensity is represented by a product of acoustic pressure and particle velocity. The acoustic pressure level can be measured easily, while the particle velocity cannot. Therefore, in general, there has been used a method (P-P method) that uses phase-matched two omnidirectional microphones which are arranged facing or back-to-back each other to determine the particle velocity through a finite difference approximation. However, the handling of the P-P method is not easy, because the P-P method is sensitive to a difference in the sensitivity or phase between the two microphones and the strict management for the interval between the microphones is required. Further, in the P-P method, the interval between the microphones needs to be changed depending on the frequency of sound from a sound source.
To solve the above problems, the present inventor has developed various types of acoustic measurement devices employing a method (C-C method) that uses a plurality of unidirectional microphones arranged in 180 degrees opposite direction in terms of directionality to measure the acoustic intensity using the directional information of the microphones. For example, Patent Document 1 discloses a device capable of calculating a sound source direction and a sound source level using a database of a level difference between microphones arranged in 180 degrees opposite direction.
The present inventor has also developed an acoustic measurement device of the C-C method that uses a reception section including a pair of unidirectional microphones which are arranged in 180 degrees opposite direction on the respective axes of an orthogonal coordinate system to perform predetermined calculation to thereby detect the direction of sound emitted from a sound source without using the database, etc (Japanese Patent Application 2007-054909).
Such an acoustic measurement device of the C-C method can solve an inherent mismatch in the phase characteristics existing between the microphones or dependency of the interval between the microphones on the frequency.    Patent Document 1: International Publication No. 2006/054599 pamphlet